I. Field of Invention
The present invention relates to processes for making radiation cured silicone rubber articles, and, in particular, to a process wherein articles formed of silicone rubber are treated substantially immediately after their formation to withstand without serious deformation the stresses involved in the mechanical handling necessary to convey such articles through irradiation apparatus.
II. Summary of the Prior Art
Unlike some of the organic rubbers, particularly the newer thermoplastic rubbers, silicone rubber must be crosslinked or vulcanized in order to have useful properties. The crosslinking process is usually referred to, in the case of silicone rubber as "cure" or "curing." Since the invention of silicone rubber, the cure has been done by incorporating free radical producing catalysts, typically organic peroxides or azo compounds, mixed with rubber and heating the composition to a high temperature, typically 150.degree.-250.degree. C., for periods ranging from a few minutes to several hours.
Virtually all commercial, heat-cured silicone rubbers today are cured with organic peroxides such as benzoyl peroxide, dichlorobenzoyl peroxide, tert-butyl perbenzoate or dicumyl peroxide. However, products cured by such curing agents all suffer from a disadvantage in that, after cure has been completed, the products contain chemical residues from the decomposition of the peroxide, and these residues tend to affect deleteriously properties such as heat-ageing, electrical resistivity and reversion resistance. The presence of these residues, moreover, limits the use of otherwise biologically inert silicone rubber products in medical applications, since the residues tend to be leached out of the rubber by body fluids, saline solutions and other liquid products used in medicine.
There are other problems of peroxides curing catalysts. One is that they are very sensitive to catalyst "poisons" and many common rubber compounding ingredients, which otherwise would be included in silicone rubber to improve physical strength, flame retardancy, thermal ageing, and so on, cannot be used because they prevent or retard the peroxide cure. For example, most reinforcing carbon blacks, commonly used in organic rubbers, completely prevent peroxide cure of silicones. Certain peroxide residues, being volatile, cause porosity in the cured rubber and others tend to diffuse out of the compound forming unsightly oily films or crystals on the surface.
More recently, liquid silicone rubbers have been introduced to the market, which cure by other mechanisms to solid, vulcanized rubber products. These liquid rubbers (which occasionally are pastes or semi-solids) typically cure at room temperature, or at relatively low temperatures, and are categorically referred to as "Room Temperature Vulcanizing" (RTV) silicone rubbers. There are several mechanisms for curing RTV rubbers. Some contain crosslinking agents which are activated by atmospheric moisture and others are cured by complex catalysts containing platinum salts. All of them yield chemical byproducts from the curing reaction, some of them relatively toxic chemicals such as organic amines, acetic acid or methanol. They all, therefore, suffer from the disadvantages cited for peroxide cure, above.
It has long been known that these disadvantages can be avoided, and that cured silicone rubber parts free of deleterious catalyst residues can be obtained, by irradiation with high energy electrons or gamma rays, to effect the cure. For example, the curing of silicone rubber with electrons was disclosed and claimed in U.S. Pat. No. 2,763,609 to Lewis, et al. This curing process and the benefits resulting therefrom are discussed in detail in that patent, the disclosure of which is incorporated herein by reference. In spite of the significant advantages of the irradiation curing process, however, it has never been commercialized, to the best of our knowledge, because of a serious obstacle to production of commercial quantities of formed, irradiated silicone parts. The problem is that Silicone rubber, like other curable polymers, must usually be formed into the desired, final shape before cure because, after cure, it is crosslinked and, therefore, not formable by the usual processes such as molding, extrusion, or casting.
Yet unlike most other polymers, silicone rubber is almost completely lacking in physical strength before cure. Uncured silicone rubber is extremely soft, easily deformed even by working with the bare hands, and it flows readily under low pressures, such as might be experienced when winding a silicone tube or sheet on a reel. Cure, whether by peroxide catalysts or by irradiation, improves the physical strength of silicone rubber dramatically. Tensile strength, for example, is increased 20-50 times.
Because of its low precure strength (referred to in the trade as "green strength"), it is impossible to convey extruded silicone tubing, or extruded silicone insulated wire, for example, from the extruder to the radiation vault, and to convey it under the electron beam repeatedly, without serious physical damage. If the silicone rubber is to be formed by molding, it is nearly impossible to remove it from the mold without damage, in order to irradiate it. Of course, it cannot be radiation-cured while still in the mold, at least by electron irradiation, because the electrons cannot penetrate the thick metal walls of a typical mold. This problem has prevented commercialization of the radiation cure process.